Method of making an acoustical transducer

ABSTRACT

A method of constructing an acoustical transducer in roller shape wherein the transducing structure is a plurality of piezoelectric segments about the circumference of the roller. The method includes the precision boring to size of the inside and outside diameters of an oversize piezoelectric tube which has been fired but is still in the unpolarized state. An electrical coating is then deposited on the inside and outside diameter and the tube is electrically polarized. The tube is filled with a low exotherm electrically conductive acoustical damping material which is allowed to harden. The tube is then divided into a plurality of parallel sections and cut into several individual roller acoustical transducers, each transducer having a plurality of individual segments.

.XF Z 396989051.

m Sta? Miller [54] METHOD OF MAKING AN ACOUSTICAL TRANSDUCER [72]Inventor: Darrow L. Miller, Los Angeles,

Calif.

[73] Assignee: North American Rockwell Corporation [22] Filed: Feb. 6,1970 [21] Appl. No.: 9,239

[52] US. Cl. ..29/25.35, 29/594, 310/81, 715/715 [51] Int. Cl. ..H04r17/00 [58] Field of Search ...29/25.35, 594; 73/678, 67.85,

[56] References Cited UNlTED STATES PATENTS 2,420,864 5/1947 Chilowsk y..29/2s.3s 2,752,662 7/1956 Crooks et al. ..29/25..35 2,846,874 8/1958Horn ..73/67.8 X 3,177,382 4/1965 Green ..3l0/8.7

[451 Oct. 17, 1972 Cowan ..29/594 UX Cook et al. ..29/600 X PrimaryExaminer-R. Spencer Annear Attorney-L. Lee Humphries 5 7] ABSTRACT Amethod of constructing an acoustical transducer in roller shape whereinthe transducing structure is a plurality of piezoelectric segments aboutthe circumference of the roller. The method includes the precisionboring to size of the inside and outside diameters of an oversizepiezoelectric tube which has been fired but is still in the unpolarizedstate. An electrical coating is then deposited on the inside and outsidediameter and the tube is electrically polarized. The tube is filled witha low exotherm electrically conductive acoustical damping material whichis allowed to harden. The tube is then divided into a plurality ofparallel sections and cut into several individual roller acousticaltransducers, each transducer having a plurality of individual segments.

16 Claims, 4 Drawing Figures PATENTEDBBIWIBYZ 3.698.051

MALE? DXl/PPQW 4.

I N VEN T OR.

Q G) 3 wmifi METHOD OF MAKING AN ACOUSTICAL TRANSDUCER BACKGROUND OF THEINVENTION The nondestructive testing of manufactured articles to locatestructural defects therein is widely practiced in modern industry. Suchtesting is of particular importance in high speed aircraft and spacevehicles wherein use of composite panels, either laminates or sandwichtype, is widespread. Such composite structures often exhibit strengthcharacteristics equivalent to solid material, while being of asubstantially lighter weight. To maintain the strength of the completedstructure, it is necessary to detect any areas in which a satisfactorybond has not occurred. Many structural components in the field such ascomposite panels in an aerial vehicle cannot be conveniently inspectedby conventional ultrasonic methods after complete and perpermanentinstallation of the panels. Also, since disbonds may occur in compositepanels periodically during the service life of an aircraft it isnecessary to inspect the aircraft structure not only during it initialfabrication but many times thereafter between flights. it should benoted that some supersonic aircraft contain as much as 20,000 squarefeet of bonded panels in their structure, whereby periodic inspectioninvolves a considerable and almost continuous effort. 7

Heretofore, it has been found most desirable to employ an ultrasonicprobe to discover disbonds in composite structures in a non-destructivemanner. Ultrasonic probes when employed with conventional 0 within thestructural surface of an aircraft. Additional electronic equipmentproduce a pulse of ultrasonic energy which is transmitted within thestructure of the workpiece. The energizing signal is reflected and/orattenuated upon coming into contact with any discontinuity within theworkpiece, and this reflection or change in receiving signal amplitudeis picked up by the probe and displayed by some conventional means suchas an oscilloscope. The acoustical pulse is usually produced by theapplication of the piezoelectric effect. The piezoelectric effectsrefers to the property of certain materials of a crystalline structure(typically barium titanate, lead zirconate, lead metaniobate) tophysically vibrate upon application of a voltage gradient theretoproducing a pulse of ultrasonic energy. The crystal structure of theprobe physically deforms and produces pulses at a rate coinciding withvariations in the excitation signal. The resultant electric pulses aretransmitted to the workpiece and any discontinuities within theworkpiece are detected by the reflection and/or attenuation of thereturn signal.

The most common type of acoustical probe used for ultrasonic inspectionhas been typically of a flat disc form. To effect the efficienttransmission of the electric pulse into the workpiece and accuratesensing of the reaction, the flat disc probe typically requires the useof a liquid or a paste couplant which wets the workpiece surface and theprobe itself thereby coupling the workpiece and the probe togetheracoustically. The probe is then moved translationally across theworkpiece surface in sliding contact relationship with the interveningfilm of couplant. Any disbonds located within the workpiece are thendetected by analysis or comparison of each energizing pulse with itscorresponding reflected pulse.

disadvantages of the wet coupling method are that the couplants aremessy, costly, involve risk of corrosion, contamination and usuallyleave residual traces.

To overcome the disadvantages of the wet coupling method, a roller typeof probe has been employed as disclosed in my co-pending applicationSer. No. 832,568, filed June 12, 1969. Roller type probes as suggestedin the stated patent application are designed in the shape of acontinuous surface cylinder of piezoelectric material. As the rollerincurs only a line contact with the workpiece surface, such rollersfacilitate the inspection of rough or high friction materials. To avoidthe use of a wet couplant in the employment of the roller probe, a drycouplant has been employed as disclosed and claimed in my said patentapplication. It has been found that such a satisfactory dry couplant maybe achieved by a glass cloth upon which is applied an adhesive orsilicon resin to which are adhered a layer of glass beads about fivemicrons in diameter. Such a dry couplant forms a good mechanical bondwith the workpiece and permits efficient energy transmission in the samemanner as the wet couplant.

Roller tubes of the mentioned type have not been difficult or costly tomanufacture. The roller tube or hoop configurations operate at a lowrange resonant frequencies (10 KHz to 500 KHz), where either the tubethickness mode or the radial mode of vibration resulted in a tubegeometry with a thick cross section relative to the tube diameter suchas a nominal thickness ranging from 0.125 inch to 0.250 inch and adiameter ranging from 0.5 inch to 3 inches. These tube configurationsare structurally sound and are not hard to make. Thus, present methodsof manufacturing piezoelectric materials into specific geometricconfigurations involve the dry molding of finely ground polycrystallineceramic compounds (lead-titanate or lead-zirconate materials) atpressures up to 16,000 psi into the desired high density materialconfiguration. The green ceramic forms thus achieved are cured atelevated temperatures until a porcelain-like solid device is obtained.The active surfaces are deposited with a conducting material, usuallysilver, and polarized at a high dc potential to make them piezoelectric.Because the silver ground polycrystalline ceramic compounds are pressedto a high density in the dry form, the material does not flow and onlyspecific geometric shapes and configurations can be formed, such astube, disk, rods, etc. Other forms involving compound curves can beachieved only by costly, difficult and complex grinding operations on aninitial blank. Sometimes this is accomplished in the green state (beforebaking in a high temperature kiln). If forming operations on a blank aredone, the material is extremely hard and requires diamond-faced orsilicon carbide tools for all cutting operations.

The low frequency (10 KHz to 500 KHz) continuous wave probe has anunbroken outside diameter and radiates energy throughout 360. Incontrast, a high frequency (1 MHz to MHz) segmented probe has only onesegment active. This segment is in contact with the sample. The highpulse echo frequencies employed provide extreme resolution compared withthe low frequency device.

The curved segments described above were first made by cutting eachsegment from a cured, polarized tube configuration. They are difficultand expensive to make, cannot be made in the proposed combinedtrapazoidal and curved segment shape. The wall thickness was limited toa minimum of 0.035 inches (approximately 2 MHz to provide a half-wavethickness).

SUMMARY OF THE INVENTION The method as defined by this invention relatesto the manufacture of a piezoelectric segmented roller probe which is tobe employed in rapid, non-destructive inspection for internal defectswithin both composite and solid type structures. A hollow, cylindrical,ceramic element approximately of an inch thick, similar to the type ofelement which is employed in the continuous roller type of probe, isformed and cured in the normal manner. The tube'is precision bored onthe inside to fit a mandril and held in place with a low meltingtemperature wax or similar material. The outside surface is thenprecision ground to provide a true roundness through 360 to the centralaxis leaving the tube still greatly oversize. Where the finished halfwave wall thickness is relatively thick, the tube can be groundcompletely to a half wave wall thickness and removed from the mandrel bymelting the wave. The final grinding process and remetalizing of theexterior surface delineated in a later step of the manufacturing processwould be omitted. A metallic polarization film is then deposited on theinside and outside diameter of the tubular element. The entire structureis electrically polarized to make the polycrystalline cylindricalelement piezoelectric. The tubular element is then filled with a lowexothern epoxy acoustical damping material. One such material, which hasbeen found to be satisfactory, comprises a mixture of 1 part Versamide140, l part epon I50, and powdered metal. The damping material canpreferably be placed within the tubular element in a liquid form andthen given time to cure or it possibly can be premolded and fixed inposition by a liquid adhesive. Using the precision ground exterior ofthe tube as a reference, a central axis is then bored lengthwide throughthe core material. This axis will be used upon completion of the probeto facilitate its rotation upon the workpiece. The combination of thecylindrical element and the core material is placed within anappropriate apparatus and the cylindrical ceramic element precisionground with a silicon carbide J l l or J12 tool to the desired half-wavethickness. Upon achieving the desired cylindrical wall thickness, ametallic film is deposited on the outside surface thereof. Thepiezoelectric material of the cylindrical element is then slottedlongitudinally to form a plurality of spaced slots formatin g longseparate parallel sections. The slot is cut slightly deeper than thepiezoelectric material is thick. The number of sections depends on thediameter and other design considerations. For a 1% inch diameter tube,sections might be employed.

The slotting may be accomplished in a direction parallel to the boredaxis hole or it may be accomplished at an angle with respect thereto. Ifthe slotting is accomplished at an angle, a plurality of parallelogramshaped sections result instead of substantially rectangular shapedsections. Upon completion of the slotting procedure, the completedassembly is then cut into a predetermined number of individualpiezoelectric roller transducers of a wheel-like shape. Each rollerhaving a number of individual piezoelectric segments spaced about itscircumference. The completed transducers can be provided with a layer ofadhesive elastomers exteriorly thereof with a layer of particulatesbeing secured to the outside surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of thecylindrical piezoelectric element having the internal core assemblyduring an intermediate step of fabrication of the roller transducer ofthis invention;

FIG. 2 is a perspective view of the roller probe of this invention whichhas been slotted in a direction parallel to the axis of the cylindricalprobe;

FIG. 3 is a view similar to FIG. 2 wherein slotting is accomplished inan angle with respect to the axis of the probe resulting inparallelogram shaped probe segments; and I FIG. 4 is a perspective viewof a completed probe assembly of the embodiment shown in FIG. 3 whichhas been installed within appropriate structure to effect movement andoperation of the probe upon a workpiece.

DETAILED DESCRIPTION OF THE INVENTION Referring particularly to thedrawing and specifically to FIG. 4, the apparatus formed by the methodof this invention comprises a device for nondestructive testing orinspection which may illustratively take the form of a manually operatedprobe 10 having a rotatable portion 12 being supported by support rod14. Probe 10 is shown in operative relationship with a workpiece of alight-weight composite panel 16 having upper and lower face sheets 18and 20, respectively, with a lightweight core material 22 securedtherebetween. Probe 10is energized by an electrical connection 24 with asuitable source 26 which may take various forms known to the prior art,the details of which are beyond the scope of the invention defined bythis application.

Support rod 14 is rotatively secured within central opening 30 of rollercore 38. Attached to the outer extremity of support rod 14 is a handle32 to facilitate manual grasping and movement of the probe 10. It is tobe understood that an inspection of composite panel 16 is performed bytraversing one surface of the panel 16, such as face sheet 16 withrotatable portion 12, by application of force to handle 28 of probe 10.The action thus achieved is far more rapid and effective than thesliding movement of a flat probe such as known in the prior art,particularly when the contacting surface of a given workpiece is rough,uneven, curved, or characterized by high friction materials.

It will be further understood that acoustical transmission betweencrystal surface 34 and the composite panel 16 may be significantlyenhanced by the use of one or more couplants many of which are known andwidely used in industry. Although the probe 10, formed by the method ofthis invention will work satisfactorily when employed with many of thewet couplants presently employed in the prior art, it is the intentionof applicant that the probe is to be employed with a dry couplant mediacomprising a layer of adhesive elastomer upon which has been applied alayer of particulates.

To form the roller probe by the method of this invention, a hollow,cylindrical crystal surface 34 is formed of a ferro-electric ceramicmaterial of the type commonly used in acoustical probes and the like.Such material is characterized by a propensity to distort or deform uponapplication of an electrical signal thereto, such deforming beingreferred to as the piezoelectric effect. In some instances, material 34could be composed of lead titanate, lead zirconate or other similarmaterials. The thickness of the tubular crystal structure 34 is chosento be greater than that desired in the final configuration. Upon thetubular structure being formed to its desired diameter and thickness,the crystal is then cured as by a conventional baking operation which isused to cure all ceramic materials. The tube is precision bored to fit amandrel and held in place with a low temperature wax or similarmaterial. The outside surface is then ground to form a true roundnessthrough 360 to the central axis leaving the tube still oversize. (Wherethe finished half wave wall thickness is to be relatively thick, thetube can be ground completely to its half wave wall thickness andremoved from the mandrel by melting the wax. The final grinding processand remetalizing of the exterior surface delineated in later processsteps would be omitted.) A metallic polarization coating 36 is appliedto the inside diameter of tubular crystal surface 34. The entire tubularcrystal surface 36 is then electrically polarized thereby making thepolycrystalline material of crystal surface 34 and 36 piezoelectric. Thecomposition of coating 36 is not significant except such coating must beelectrically conductive. Coating 36 will be employed to permitapplication of electrical signals to the crystal surface 34. One suchmetallic material which has proved to be satisfactory for coating 36 issilver or an alloy thereof which may be electrodeposited on the surface.

The hollow portion within the crystal surface 34 is then filled with anelectrically conductive acoustical damping material. One such materialwhich has proved to be most satisfactory would be a mixture of epoxy andpowdered metal. An opening is bored longitudinally through the corematerial 38 coinciding with the lon gitudinal axis of the cylindricalcrystal surface 34. Continuous crystal surface 34 is then reduced to thedesired wall thickness which has been predetermined. It is to beunderstood that the wall thickness determines the frequency at which theroller probe 10 of this invention operates. It is envisioned that thereducing of the wall thickness would be accomplished as by grinding offthe excess piezoelectric crystal surface material. However, anydesirable or satisfactory means to accomplish the reducing to size ofthe wall thickness of the crystal surface will be most satisfactory. Apolarizing metallic coating 40 is then applied to the exterior surfaceof crystal surface 34. The coating 40 is normally identical to coating36. Normally such coatings 36 and 40 will be applied as byelectro-depositing, however if the core has been previously filled,outgassing will occur in thisprocess and any conventional means ofapplying the film to the crystal surface 34 will be satisfactory.

It is to be noted that in some instances the reducing of the wallthickness of the crystal surface 34 may be accomplished prior to thebaking or curing step of the crystal surface 34. Once the surface 34 iscured, a substantially greater hardness is achieved and also thestructure is more brittle. Therefore, to avoid any possibility offorming defects in the surface 34 and to increase the speed of reducingthe wall thickness of the crystal 34, it may be desirable to effect thereducing of the wall thickness in this green state prior to the curingprocedure.

Referring particularly to FIG. 2 of the drawing, the crystal surface 34may then by cut radially by a plurality of longitudinal slots 42. Thedepth of each slot 42 extends within the core material 38 therebyseparating crystal surface 34 into a plurality of separate segments 44.Slots 42 are in a direction parallel to central opening 30. The exactnumber of segments 44 is to be considered to be a matter of choice ordesign, and the method of this invention should in no way be limited asto the specific number of such segments. Once these segments 44 havebeen formed, the tubular crystal surface 34 is then sliced as shown byphantom line 46, thereby forming a roller probe of the desired width (asin FIG. 4). It is to be understood that a number of probes can be formedfrom each tubular crystal.

Referring to FIG. 3 of the drawing, radial slots 48 divide the crystalsurface 34 into a plurality of segments 50. It is to be noted that slots48 are not parallel to central opening 30 but extend at an angle withrespect thereto. As a result, each of the segments 50 thus formed arenot substantially rectangularly shaped as in the embodiment of FIG. 2but are in the shape of a parallelogram. Each of the slots 48 alsoextend within core material 38 as slots 42. The longitudinal crystalsurface 34 is then sliced into a plurality of roller probes having thedesired width as represented by phantom line 52. The desirability of theparallelogram shaped segments as shown in FIG. 3 over the rectangularshaped segments as shown in FIG. 2 is that, as the rotatable portion 12of the probe 10 is moved upon the workpiece surface, contact with theworkpiece is in a line direction parallel to central opening 30.Therefore, as one of the slots 48 comes into contact with the workpiecesurface 18 the fore end of one segment 50 is in contact with theworkpiece surface and also the aft end of an adjacent segment is also incontact with the workpiece surface 18. Therefore, a continuous couple iscreated which is not influenced by the gaps of slots 48. In other words,a more efficient contacting relationship between the rotatable portion12 is effected between the segments 50 and the workpiece surface 18.

It is to be noted that the contacting relationship of both the segments44 and 50 is in essence a line with the upper face sheet 18 of thecomposite panel 16.

It is to be understood that a couplant must be employed between thesegments 44 or 50 and the workpiece surface 18 to effect the efficientacoustical energy transmission therebetween. Although either wet or drycouplants could be employed, it has been found in the past to be mostdesirable to employ the use of a dry couplant such as has been discussedpreviously in this application. lllustratively, one desirable form ofsuch a dry couplant includes the use of an adhesive elastomer upon whichis secured a layer of particulates as glass beads of an extremely smalldiameter (about microns). It is to be understood that the couplantitself forms no part of the method of this invention, it merely beingdisclosed to complete the description of the roller probe thus formed bythe method of this application, such a couplant being necessary toeffect use of the roller probe manufactured by the method of thisinvention.

The electrical connections to the individual segments 44 and 50 are notdescribed within this application. It is to be understood that theelectrical pulse causing each segment 44 and 50 to deform due to thepiezoelectric effect is transmitted to the segment through coating 36and 40. The electrical connections with the coatings 36 and 40 can beaccomplished by a conventional commutating arrangement. Such acommutating arrangement is readily known in the art and need not bedescribed here.

I claim:

1. A method of making an ultrasonic probe for nondestructive inspectionof a workpiece to locate internal structural defects therewithin, saidmethod including:

forming a piezoelectric material into a hollow cylinder;

curing said cylinder to harden the same into a tubular crystalstructure;

applying a metallic coating on the interior and exterior of saidcylinder;

filling the interior portion of said cylinder with acoustical dampingmaterial; and

dividing said tubular crystal structure into a plurality of separatearcuate segments.

2. The method of claim 1 wherein the step of dividing includes:

forming each segment substantially in the shape of a rectangle.

3. The method of claim 1 wherein the step of dividing includes:

forming each segment substantially in the shape of a parallelogram.

4. A method of making an ultrasonic probe for nondestructive inspectionof a workpiece to locate internal structural defects therewithin, saidmethod including:

forming a piezoelectric material into a hollow cylinder,

curing said cylinder to harden the same into a piezoelectric crystal,

applying a metallic coating on the interior and exterior surfaces ofsaid cylinder,

filling the interior of said cylinder with acoustical damping material,

dividing said cylindrical crystal structure into a plurality of separaterings, and

slotting each of said rings into a plurality of individual segments.

5. The method of claim 4 wherein the step of slotting includes:

forming each segment substantially in the shape of a rectangle.

6. The method of claim 4 wherein the step of slotting includes:

forming each segment substantially in the shape of a parallelogram.

7. A method of making a probe usable to nondestructively inspect aworkpiece to locate internal structural defects therewithin; said methodincluding:

forming a tubular cylindrical crystal structure which is capable ofexhibiting the piezoelectric effect; applying a metallic polarizationcoating on the interior cylindrical surface of the crystal structure;filling the interior portion of the crystal structure with acousticaldamping material; applying a metallic polarization coating on theexterior cylindrical surface of the crystal structure; and

slotting the crystal structure lengthwise to form a plurality ofacoustically separated piezoelectric segments.

8. The method as defined in claim 7 wherein the step of slottingincludes:

locating the slots at an angle to the central longitudinal axis of thecrystal structure thereby forming parallelogram shaped arcuate segments.

9. The method as defined in claim 7 wherein:

the step of forming includes selecting a crystal structure of a greaterthickness than desired; and

after the step of filling, reducing the crystal structure in thicknessto a desired minimum thickness.

10. The method as defined in claim 9 wherein:

the crystal structure is cured to the desired hardness prior to thereducing in thickness, and

said reducing in thickness is by grinding said cured crystal structure.

1 l. The method as defined in claim 9 wherein:

the crystal structure is cured to the desired hardness after saidreducing in thickness.

12. The method as defined in claim 9 wherein the step of slottingincludes:

locating the slots at an angle to the central longitudinal axis of thecrystal structure thereby forming parallelogram shaped arcuate segments.

13. A method of making a probe usable to nondestructively inspect aworkpiece to locate internal structure defects therewithin, said methodincluding:

employing a tubular cylindrical crystal structure of ferroelectricceramic material of a thickness greater than desired;

applying an electrically conductive coating upon the interior surface ofthe crystal structure;

filling the interior portion of the crystal structure with an acousticaldamping material thereby forming a central core;

boring an axis centrally located longitudinally within the core;

reducing the wall thickness of the crystal structure to the desiredvalue of thickness;

applying an electrically conductive coating upon the exterior surface ofthe crystal structure; electrically polarizing the crystal structuremaking the structure piezoelectric;

dividing the crystal structure into a plurality of acousticallyseparated segments; and

cutting the crystal structure and core into a plurality of individualprobes.

14. The method as defined in claim 13 wherein:

the step of dividing includes forming the segments substantially in theshape of a parallelogram.

15. The method as defined in claim 14 further including:

1. A method of making an ultrasonic probe for non-destructive inspectionof a workpiece to locate internal structural defects therewithin, saidmethod including: forming a piezoelectric material into a hollowcylinder; curing said cylinder to harden the same into a tubular crystalstructure; applying a metallic coating on the interior and exterior ofsaid cylinder; filling the interior portion of said cylinder withacoustical damping material; and dividing said tubular crystal structureinto a plurality of separate arcuate segments.
 2. The method of claim 1wherein the step of dividing includes: forming each segmentsubstantially in the shape of a rectangle.
 3. The method of claim 1wherein the step of dividing includes: forming each segmentsubstantially in the shape of a parallelogram.
 4. A method of making anultrasonic probe for non-destructive inspection of a workpiece to locateinternal structural defeCts therewithin, said method including: forminga piezoelectric material into a hollow cylinder, curing said cylinder toharden the same into a piezoelectric crystal, applying a metalliccoating on the interior and exterior surfaces of said cylinder, fillingthe interior of said cylinder with acoustical damping material, dividingsaid cylindrical crystal structure into a plurality of separate rings,and slotting each of said rings into a plurality of individual segments.5. The method of claim 4 wherein the step of slotting includes: formingeach segment substantially in the shape of a rectangle.
 6. The method ofclaim 4 wherein the step of slotting includes: forming each segmentsubstantially in the shape of a parallelogram.
 7. A method of making aprobe usable to nondestructively inspect a workpiece to locate internalstructural defects therewithin; said method including: forming a tubularcylindrical crystal structure which is capable of exhibiting thepiezoelectric effect; applying a metallic polarization coating on theinterior cylindrical surface of the crystal structure; filling theinterior portion of the crystal structure with acoustical dampingmaterial; applying a metallic polarization coating on the exteriorcylindrical surface of the crystal structure; and slotting the crystalstructure lengthwise to form a plurality of acoustically separatedpiezoelectric segments.
 8. The method as defined in claim 7 wherein thestep of slotting includes: locating the slots at an angle to the centrallongitudinal axis of the crystal structure thereby forming parallelogramshaped arcuate segments.
 9. The method as defined in claim 7 wherein:the step of forming includes selecting a crystal structure of a greaterthickness than desired; and after the step of filling, reducing thecrystal structure in thickness to a desired minimum thickness.
 10. Themethod as defined in claim 9 wherein: the crystal structure is cured tothe desired hardness prior to the reducing in thickness, and saidreducing in thickness is by grinding said cured crystal structure. 11.The method as defined in claim 9 wherein: the crystal structure is curedto the desired hardness after said reducing in thickness.
 12. The methodas defined in claim 9 wherein the step of slotting includes: locatingthe slots at an angle to the central longitudinal axis of the crystalstructure thereby forming parallelogram shaped arcuate segments.
 13. Amethod of making a probe usable to nondestructively inspect a workpieceto locate internal structural defects therewithin, said methodincluding: employing a tubular cylindrical crystal structure offerroelectric ceramic material of a thickness greater than desired;applying an electrically conductive coating upon the interior surface ofthe crystal structure; filling the interior portion of the crystalstructure with an acoustical damping material thereby forming a centralcore; boring an axis centrally located longitudinally within the core;reducing the wall thickness of the crystal structure to the desiredvalue of thickness; applying an electrically conductive coating upon theexterior surface of the crystal structure; electrically polarizing thecrystal structure making the structure piezoelectric; dividing thecrystal structure into a plurality of acoustically separated segments;and cutting the crystal structure and core into a plurality ofindividual probes.
 14. The method as defined in claim 13 wherein: thestep of dividing includes forming the segments substantially in theshape of a parallelogram.
 15. The method as defined in claim 14 furtherincluding: applying a dry couplant about the exterior electricallyconductive coating.
 16. The method as defined in claim 15 furtherincluding: providing means to apply an electrical charge to the interiorelectrically conductive Coating; and providing means to receive anelectrical charge from the exterior electrically conductive coating.